Radar sensors are increasingly being used in motor vehicles in order to sense the traffic environment in the context of driver assistance systems, for example for radar-assisted adaptive cruise control (ACC). A certain angular resolution capability is achieved by the use of a multi-beam radar sensor, so that for each vehicle that is detected, a decision can be made as to the lane in which it is located.
In multi-beam radar sensors that are common at present, multiple antenna patches that each generate a beam are disposed next to one another in the focal plane of a refractive lens that has a refractive index suitable for microwaves, so that the radar beams are concentrated as in the case of an optical lens. The location of the antenna patch determines the direction in which the respective radar beam is emitted. In a monostatic antenna concept, the radar signal reflected from the detected objects is focused by the same lens back onto the patch that generated the beam. These known radar sensors are relatively bulky, since their installation depth must correspond approximately to the focal length of the lens.
German Published Patent Application No. DE 199 51 123 discloses a radar sensor of the aforesaid kind in which a planar group antenna is used as an antenna, and a planar lens (a so-called Rotman lens) is used instead of the refractive lens. This makes possible a much thinner design.
The group antenna has antenna elements disposed in matrix form in rows and columns. The antenna elements in each vertical column are preferably triggered at least approximately in-phase, thus resulting in beam shaping vertically. The Rotman antenna is a planar microwave guide having multiple outputs that are each connected to one column of the antenna elements via delay lines of different lengths. The geometry of the planar microwave guide and the lengths of the delay lines are selected so that a phase relationship between the antenna columns is obtained by way of different signal transit times within the lens, thus providing beam shaping horizontally and a desired directional characteristic. Because the Rotman lens moreover has multiple inputs, it is possible, by selecting the input, to determine the direction in which the principal lobe of the radar beam is emitted. The radar lobe can thus be pivoted horizontally by injecting the radar signal successively through different inputs of the lens, so that the entire detection angle region of the radar sensor can be scanned during one complete swing. In this case, therefore, only a single transmit and receive system, with a single mixer, is required for the multiple radar beams.
In a publication by A. F. Jacob, C. Metz, J. Grubert, J. Heyen, and L. C. Stange entitled “Advanced Radar Concepts for Automotive Applications,” IEEE MTT-S International Microwave Symposium IMS 2002, this concept is compared with an alternative concept in which all the radar beams are generated simultaneously, and the radar echoes from the various directions are received simultaneously. Here a separate transmit and receive device, with its own mixer, is required for each beam. An advantage of this concept, however, is the fact that because of the fixed phase and amplitude relationship between the individual beams, the angle information contained in the radar echoes can be extracted—by time-synchronized scanning (digitizing) of intermediate-frequency signals of the individual beam lobes—with no need for a particular phase reference. A further advantage is that a complete radar measurement in the entire detection angle region can be performed within a shorter cycle time, and/or a longer measurement time is available for the individual measurement; in the case of a frequency modulated continuous wave (FMCW) radar, for example, this allows improved resolution in determining the distances and relative velocities of the detected objects.
The function of the mixer system is to mix the received radar signal, whose frequency is on the order of, for example, 77 GHz, with a high-frequency signal of a local oscillator, so that the mixing yields an intermediate-frequency signal suitable for further evaluation.
In a homodyne mixer system, a portion of the signal of an oscillator that generates the signal to be transmitted is diverted and used as a local oscillator signal, so that the transmitted signal and the signal of the local oscillator have the same frequency. The intermediate frequency then corresponds to the frequency difference between the local oscillator signal and the received signal, and in a Doppler radar depends on the Doppler shift. In an FMCW radar, the frequency of the transmitted signal, and consequently the local oscillator frequency as well, are modulated in ramp form. The intermediate frequency then also depends on the signal transit time and thus on the distance of the detected object, and is on the order of from 0 to a few 100 kHz.
In a monostatic system, the signal that is received by the antenna and is to be sent to the mixer must furthermore be separated from the transmitted signal that is traveling on the same line to the antenna. This can be done, for example, using a circulator that on the one hand conveys the signal coming from the oscillator almost losslessly to the antenna, and on the other hand conveys the signal coming from the antenna almost losslessly to the mixer. In a context of parallel processing of the signals from multiple radar beams, a separate circulator would then be needed for each channel. This solution is hardly practical for motor vehicles, however, because of the space requirements and the relatively high cost of the circulator.
A variety of other homodyne mixing concepts are known, which nevertheless generally require acceptance of certain power losses.
In combination with a push-pull mixer constituted by a coupler and two diodes, for example, it is possible to use a further coupler, e.g. a ring hybrid coupler or a 90-degree hybrid coupler, that splits the signal coming from the oscillator into two portions, of which one is conveyed to the antenna and the other is conveyed as a local oscillator signal to the mixer, and that at the same time splits the signal received from the antenna into two portions, one of which is conveyed to the mixer. The other portion of the received signal, however, is fed back into the oscillator output and is therefore lost.
In the case of an unbalanced mixer having only a single nonlinear diode, a Wilkinson splitter can also be used instead of the coupler.
The previously mentioned publication of Jacob et al. describes a mixing concept with a coupler in which an out coupled portion of the signal coming from the oscillator is annihilated in a terminating resistor.
German Published Patent Application No(s). DE 196 10 850 and DE 102 35 338 describe examples of so-called transfer mixers in which the signal coming from the oscillator is conveyed exclusively to the mixer. The mixer is in this case tuned so that a portion of the oscillator signal is passed through the mixer to the antenna; this type of mixer is therefore also referred to as a “blowthrough” mixer.